The little "cube" it connects to is an aftermarket circuit breaker, as used perhaps for electric brakes or a 12V feed to a trailer connector. Based upon proximity to the battery and the 3/8" ring size, I suspect the dangling end should connect to the battery + IF YOU WANT to feed whatever that thing goes to.
I agree with the other guys. Its a big enough lug that it could have gone to the battery to feed something that was added on. Let us know what it was if you figure it out. Trailer brakes would be kinda dumb since there is a pair of factory wires in the harness and a couple empty fuse holders and suds to connect them too in the underhood fuse box that is just a few inches from there. If it wasn't trailer brakes and the truck doesn't have any its still dumb because then those 2 fuses would be totally available to do a much cleaner and more reliable install of what ever it was.
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